In an 8- to 10-quart heavy pot, combine lemon-strawberry mixture and sugar. Bring to a full rolling boil, stirring constantly. Quickly stir in pectin. Return to a full rolling boil, stirring constantly. Boil hard for 1 minute, stirring constantly. Remove from heat. Quickly skim off foam with a metal spoon.

Process filled jars in a boiling-water canner for 5 minutes (start timing when water returns to boiling). Remove jars from canner; cool on wire racks. Allow to set at room temperature for 2 weeks before serving.